Adding or subtracting to date field in WHERE clause

I am trying to set up a few different queries based on a date field and have no idea how to do it.  For example, I need to get a date 4 months before the date field.  I also need to do 2 weeks before.  So,

SELECT * FROM Oppo WHERE [TODAY'S DATE IF 4 MONTHS BEFORE oppo_finishdate ] or
SELECT * FROM Oppo WHERE [TODAY'S DATE IF 2 WEEKS BEFORE oppo_finishdate ]

How do I setup these queries?
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